Precious metal extraction involves the use of compounds called lixiviants to leach (or lixiviate) precious metals from ore. A commonly used lixiviant of the last century is cyanide. Cyanide is a highly toxic compound that is capable of causing tremendous damage to the environment. If cyanide pollutes water sources, it can kill local livestock and people. Processes that use cyanide are costly due to the requirement that all effluents that contain cyanide must be cleaned up prior to their release. Due to this costliness combined with public pressure for more environmentally-conscious processes, the use of lixiviants other than cyanide is being investigated. Such alternative lixiviants include thiocyananates, thioureas, halogens, and thiosulfates.
Thiosulfate is a promising replacement for cyanide in the precious metal extraction industry since thiosulfate lixiviants are safer and less expensive than toxic cyanide. Leaching gold from gold ore using thiosulfate leaching solution provides fast reaction rates, a high recovery of gold into the leaching solution, and a low unit cost (Muir, 2001). Gold dissolution in thiosulfate solution in the presence of a copper-ammonia catalyst can be as fast as cyanidation (Thomas, 1998). Thiosulfate leaching is particularly effective relative to other lixiviants, for leaching gold from carbonaceous ores.
Recovery of dissolved gold from thiosulfate leaching solutions can be problematic. Currently, techniques to recover gold from thiosulfate leachates include activated carbon adsorption, resin exchange, metal cementation, non-metal precipitation and solvent extraction. Problems that are associated with these methods include: gold thiosulfate complexes exhibit a low affinity for activated carbon surfaces; some resin surfaces are inhibited by degradation products of thiosulfate (Fleming, 2003; Nicol et al., 2001); and cementation techniques that use zinc, copper, iron and aluminum involve high reagent costs and both their recovered solid products and their barren solutions may be contaminated with these metals. A copper precipitation method has been described in U.S. Pat. No. 5,354,359, wherein fine copper particles are used to precipitate gold. This method can suffer from high reagent cost, incomplete recovery and complications caused by dissolved copper.
A gold recovery process is described in U.S. Pat. No. 4,913,730 wherein gold is recovered from pregnant aqueous solution by contacting the solution with hydrogen gas until the gold precipitates. This process reduces the gold ion to gold of oxidation state zero (elemental gold). The requirement to have hydrogen gas present in large quantities in remote areas is difficult and can be a safety hazard.
Thus there is a need for a method that can economically and efficiently recover precious metals such as gold and silver from thiosulfate leaching solutions.